The estimation of an external exposure to formaldehyde in tobacco smoke requires a reliable analytical method. The gas chromatographic determination of formaldehyde shows in comparison to photometric methods a higher sensitivity and specificity. In the main stream smoke of various kinds of cigarettes the amount of formaldehyde vary between 3.4 micrograms to 8.8 micrograms/cigarette, this is equal to concentration between 2.3 to 6.1 ppm. In the air of lounges in hospitals formaldehyde concentrations up to 0.19 ppm can be detected after smoking of 15 cigarettes over a period of 1.5 h. In kindergartens without tobacco smoke the formaldehyde concentrations in air range from 0.005 to 0.01 ppm. The smoking of 30 cigarettes and one pipe in a non ventilated room over 1.5 h exceeded formaldehyde concentrations between 0.21 to 0.45 ppm. The concentration declines to 0.08 ppm within 2h after termination of smoking. The MAK-value of 0.5 ppm is not exceeded; in contrast to this the indoor limit of 0.1 ppm recommended by the German Bundesgesundheitsamt is exceeded in the vicinity of the smoker. The formaldehyde concentrations in tobacco smoke reported in the older literature can not be confirmed; this is due to the nonspecificity of the photometric methods. On the basis of our results can be concluded that the irritating effects of tobacco smoke to the mucous membranes are the result of the sum of irritating effects caused by several compounds and particles in the smoke and not only the impact of formaldehyde.